Many hoisting devices for raising or lowering an object between a platform and a vertically moving surface have recognized the need to prevent the occurrence of slack in the hoisting cable. If slack is allowed to develop in the cable, the motion of the object as it rests on the moving surface will cause a violent jerk as the slack is taken up. This jerk may cause undue stresses to the hoisting cable and supporting structure, damage the hoisting connections of the object and the hoisting device, or discomfort to passengers in the object if a lifeboat.
Prior art devices as in U.S. Pat. Nos. 2,402,784 and 2,178,305 are attempts to prevent the occurrence of slack immediately prior to raising the object from the surface to the platform by creating a constant tension condition in the cable. U.S. Pat. No. 2,178,305 further switches from this constant tension mode to a hoisting mode at an optimum point in the waveform of the moving surface. These devices are expensive and complicated and are not well suited to lowering an object from the platform to the moving surface.